This invention pertains generally to variable valve systems for use in internal combustion engines, and more specifically to a method and apparatus to control the variable valve system during transient engine operations.
Engine manufacturers incorporate variable valve systems, including variable cam phasing systems, to improve operating and emissions performance of internal combustion engines. Distinct engine operating characteristics resulting from use of the variable valve system include improved combustion stability at idle, improved airflow into the engine over a range of engine operations corresponding to improvements in engine performance, and improved dilution tolerance in a combustion charge. Benefits of incorporating the variable valve system into an engine include improved fuel economy, improved torque at low engine speeds, lower engine cost and improved quality through elimination of external exhaust gas recirculation (EGR) systems, and improved control of engine exhaust emissions.
A typical internal combustion engine is comprised of at least one cylinder containing a piston that is attached to a rotating crankshaft by a piston rod. The piston slides up and down the cylinder in response to combustion events that occur in a combustion chamber formed in the cylinder between the piston and a head. The head contains one or more intake valves to control the flow of air and fuel into the combustion chamber, and one or more exhaust valves that control the flow of exhaust gases out of the combustion chamber. A rotating camshaft opens and closes the intake and exhaust valves, and is synchronized with the position of each piston and the crankshaft. As an example of a variable valve system, a typical variable cam phasing system includes one or more variable cam phasers attached to an engine camshaft, and a cam position sensor that measures rotational position of each camshaft. The variable cam phasing system varies the opening and closing of each intake or exhaust valve by varying angular position and rotation of the camshaft, relative to angular position and rotation of the crankshaft and each respective cylinder. An oil control valve diverts pressurized engine oil to the variable cam phaser, primarily based upon feedback from the cam position sensor. Typically an electronic engine controller controls this operation.
Timing of the intake valve opening affects mass of air that flows into an individual cylinder, thus affecting volumetric efficiency of the internal combustion engine. This also affects fuel delivery, because fuel delivery is typically determined by measuring or calculating mass air flow and determining an air/fuel ratio that is required to meet operator performance requirements and engine emissions requirements. The quantity of fuel delivered to each cylinder is determined based upon the combination of mass airflow to the cylinder and the required air/fuel ratio. A combustion charge is created in each combustion chamber by delivering the quantity of fuel near the intake valve of the cylinder, or directly into the cylinder. This is known to one skilled in the art. When the mass airflow into the cylinder cannot be repeatably predicted because of an unpredictable position of the variable cam phaser, the fuel control system may overfuel or underfuel the combustion charge, leading to problems with combustion stability, and variations in air/fuel ratio that affect emissions and driveability.
The performance of the variable cam phasing system, in terms of response time and ability to maintain a position, is affected by several factors in the system. These factors include engine oil pressure and flow; oil viscosity, age and contamination; part-to-part variability caused by manufacturing tolerances and component wear; and engine operating temperature. These factors result in an inability of a controller to precisely determine position and response time of the variable cam phaser, especially during transient engine operation. The previously described benefits derived from use of the variable cam phasing system may be compromised due to variations in response times. An engine with dual cylinder banks may also experience differences between the two banks in terms of response time of each cam phaser that is caused by differences in oil pressure and flow at each bank. This may result in further reduced engine performance due to vibration and engine instability caused by variations in bank-to-bank airflow, individual cylinder fueling, and volumetric efficiencies.
By way of example, the engine controller uses the variable cam phaser on air intake valves to open each valve early in the intake stroke to improve volumetric efficiency at low engine speeds. The result is improved engine torque at low speeds, allowing for improved vehicle acceleration. In current variable cam phasing systems, the system is calibrated based upon a known set of engine operating factors and a limited quantity of components. The controller controls the cam phasing system and the closed-loop control system and compensates for air flow variation caused by the factors previously discussed (i.e. contamination, part-to-part variability, engine operating temperature, oil viscosity, and component wear) with feedback from the cam position sensor and exhaust gas sensors, under most operating conditions. However, when the engine is engaged in a transient maneuver such as acceleration or deceleration, the lag time associated with the feedback system limits the ability of the controller to compensate for inaccuracies in open-loop airflow estimates due to variation in transient response of the variable cam phaser. The unknown instantaneous volumetric efficiency related to the resulting lag time leads to incorrect fueling of individual cylinders as well as reduced EGR tolerance in the combustion charge. The result is increased engine-out emissions, reduced engine power, increased combustion instability, and increased potential for cylinder misfire, as described previously. The engine performance problems and compromises are further exacerbated when the cylinders of the engine are in a dual bank configuration with separate variable cam phasing hardware for each bank of cylinders, as described previously. Hence, there is a need for a method and system to compensate for variability of response of a variable cam phasing system during transient engine operation.
The present invention is an improvement over conventional engine systems that employ variable cam phasing in that it provides a method and apparatus to control the rate of change of the variable cam phasing system during transient engine operating conditions. The goal of the invention is primarily to maintain combustion stability. The method includes determining a rate of change of the variable cam phasing system substantially necessary to maintain combustion stability. It monitors an operating point of the engine, and determines a desired operating point of the engine. The invention controls the rate of change of the variable cam phasing system based upon the operating point of the engine, the desired operating point of the engine, and the rate of change of the variable cam phasing system necessary to maintain combustion stability. These and other objects of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description of the embodiments.